Field of the Invention
The present invention relates to a method for color control in a digital printing machine including a control unit and a color measuring device.
As is the case with lithographic offset printing presses, digital printing machines such as inkjet printing machines or printing machines relying on toner-based technology aim at reproducing the colors of an original, today mostly a digital original, as accurately as possible. A large number of techniques have been developed for that purpose. They ensure an accurate representation of the colors by implementing a color control process in the digital printing machine. A known technique is to take measurements on a printing substrate during the printing process by using a color measuring device, to calculate potential deviations and subsequently to minimize those deviations by carrying out control operations in the ink application system of the digital printing machine. Those control operations may be minimized or even eliminated if specific printing machine properties that result in color deviations are factored in from the beginning. In most cases, that process relies on so-called color profiles, which represent a kind of a finger print of the printing machine that reflects the characteristics of a machine in terms of color accuracy. A standardized color profile is the so-called ICC color profile. However, a correct representation of the colors does not only depend on the characteristics of the machine, but also on the characteristics of the ink in the printed substrate.
International Publication WO 2008/127553, corresponding to U.S. Pat. No. 7,599,634, discloses a digital printing machine and a method for operating a digital printing machine that are compatible with a plurality of printing substrates. The disclosed digital printing machine uses toner. The method involves the use of so-called substrate-specific ICC profiles instead of a single ICC profile. A problem with that approach is, however, that the large number of different printing substrates creates the logistic problem of administering all of the substrate-specific ICC profiles. Thus, the proposal is made instead to reduce the number of substrate-specific ICC profiles to a few universally applicable color profiles and to adapt the few universal ICC profiles to the physical properties of the printing substrate that is currently in use. The substrates are, in particular, categorized as one of the four basic categories of coated/uncoated and glossy/matte. In order to adapt those universal color profiles, the most important colors are printed onto the printing substrate in question and measurements are taken on a number of color measurement strips. That avoids time-consuming new measurements on hundreds or thousands of color measurement fields to generate a dedicated color profile. In that way, the negative effect that the adjustment/calibrating process may have on the productivity of the digital printing machine may minimized.